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2O2                                    CQNDYLARTHRA                                   CHAP.

here collected it is clear that a direct effect is produced. If we
are to regard horns as secondary sexual appendages which have
been subsequently handed on to the female by heredity, we should
expect to meet with examples of animals now homed in both
sexes, of which the earlier representatives had the horns confined
to one sex. This is most interestingly shown by the extinct and
Miocene Giraffe, tSamotfaeriuin, of which the male alone had a pair
of short horns, while the skull of the female was entirely hornless;
the modern G-iraffct, as is well known, has horns in both sexes.

It is interesting to note that the existing Perissodactyles and
Axfciodactyles are to be distinguished by their unpaired or paired
horns. But while there are no Artiodactyles with unpaired horns
(save occasional sports) the Perissodactyles have more than once
tried, so to speak, paired horns, which ultimately proved fatal
to them. The Hhinoceros Diceratheriuvn apparently inherited and
improved upon the small paired horns of -Aceratherium, but it has
left no descendant. The paired horned Titanotheria offer another
instance of the same apparent incompatibility between the Perisso-
dactyle structure and the persistence of paired horns.

This group is characterised by the following assemblage of
characters. Extinct, often plantigrade Ungulates, with five-toed
limbs. Bones of carpus and tarsus not always interlocking, but
sometimes lying above each other in corresponding positions.
The burner us has an entepicondylar foramen. Dental formula
quite complete ; the molars brachyodont and bunodont. The
pretnolars are simpler than the molars. The canines are small.
As with other early types, the zygapophyses are flat and do not
interlock. The astragalus is like that of the Creodonta. This
group was American and European, in range, the remains of its
rather numerous genera beis&g of Eocene time. The best-known
genus is J?henacodus, of which some account will be given before
discussing the, in many cases, more fragmentary remains of
other allied forms.
The genus JPhenacodus was first described so long ago as 1872,
from a few scattered teeth.     Since then several nearly complete
'skeletons have been  obtained, and we are  in  full possession of